In practice it is always necessary to centre the mould sections of, for example, injection moulding machines with respect to one another in order accurately to delimit the mould cavity. It has been observed that movement of mould sections relative to one another in a direction other than the closing direction is undesirable, certainly when injecting plastic. In a wide variety of processes plastic is introduced before the mould sections are completely closed, whilst, moreover, processes exist in which the mould cavity is never completely closed. A few such are known as praegen/coining venting. In the case of coining a quantity of plastic is introduced during closure, but this does not completely fill the mould cavity. Complete filing takes place during closure as a result of the reduction in size of the mould cavity. In this case of venting the mould opens after having been closed.
In all cases movement in the radial plane with respect to the direction of closure is undesirable. This applies in particular when injecting products with high accuracy. Optical information carries, such as CDs, DVDs and the like may be mentioned as an example. As the information density increases, ever more stringent requirements are being imposed on the peaks and troughs. Moreover, it has been found that with certain plastics the clarity thereof is reduced by mutual movement of the mould sections in the radial plane.
In the state of the art as described above, centring takes place with the aid of a cone. That is to say the body is of conical construction, whilst the seat is provided with a corresponding conical opening. However, in theory such a centring functions only in one position of the mutual movement of the mould sections, that is to say the position in which these parts touch one another. If contamination is present between the cone and the seat, the cone is pushed in one direction by said contamination and accurate centring can no longer be guaranteed.
In order to eliminate this disadvantage of centring only in one position, so-called block guides have been proposed in prior art. With these guides the mould sections are centred relative to one another over a longer path of the closing movement. However, the mechanical forces needed to achieve movement in such a guide are appreciable. After all, there may not be any play between the various parts because otherwise optimum guiding cannot be guaranteed. Moreover, there is the problem that on heating one of the mould sections either the play increases or jamming takes place between the parts of the block guide moving relative to one another, as a consequence of which the wear increases appreciably.